PRAMs (phase-change Random Access Memories) have been conventionally studied in order to accomplish an ultrahigh-density memory. Recording or erasing of data is carried out with respect to the PRAMs by use of a physical property change caused by a transition (called 1st-order phase transformation) between a crystalline state and an amorphous state of chalcogen compounds containing Te (see, for example, Patent Literature 1 and Non patent Literatures 1 and 2).
An alloy thin film has been usually employed as a recording material for the PRAMs. The alloy thin film has a single layer and is deposited between electrodes by a vacuum film deposition method such as sputtering with the use of a target made from a compound composition. Because of this, the alloy thin film has a thickness of 20 nm to 50 nm and is a polycrystal, not a single crystal.
Since the latter half of the 1980s, chalcogen compounds that contain Te have been studied as to their crystal structures and amorphous structures by structure analysis using an X-ray and/or the like. Note, however, that the crystal structures of the chalcogen compounds had been unclear until 2004. This is because of the presence of Sb. Sb is one of the atoms constituting the chalcogen compound containing Te, and has the atomic number next to Te, so that the difference in the number of electrons is only one (1) between Sb and Te. This makes it almost impossible to distinguish between Te and Sb by use of the X-ray diffraction or the electron diffraction.
This caused misunderstandings as to crystal structures of (i) a compound called GeSbTe (225 composition), which has been experimentally known to have excellent properties and (ii) compounds similar to a pseudo-binary composition compound (i.e., compounds, having 225, 147, or 125 composition, similar to GeTe—Sb2Te3), which compounds (i) and (ii) are and used in rewritable optical discs which have been already on the market and have been used in rewritable optical disks. Specifically, it had been believed that the crystal structures of the compounds (i) and (ii) had sodium-chloride structures in each of which Te occupies Na sites (a-sites) and Ge or Sb occupies Cl sites (b-sites) randomly (see, for example, Non patent Literature 3).
However, the structure of the GeSbTe compound was analyzed in detail by use of a device such as a synchrotron orbital radiation device. This has revealed that the chalcogen compound that contains Te has a structure different from the structures that had been believed in the following respects (see, for example, Non patent Literature 4).
Specifically, the following facts (1) through (3) came out. (1) In a crystal phase, an arrangement in which Ge atoms and Sb atoms occupy positions ((b)-sites) of Cl in a NaCl-type simple cubic lattice is not a ‘random’ state as previously believed. Instead, positions at which the atoms are arranged are precisely ‘fixed’ and the lattice is distorted (see FIG. 3). (2) The amorphous state is not completely random but has a twisted structure, while maintaining a unit, in which Ge atoms inside the crystal lattice are arranged so as to be slightly shifted by about 0.2 Å from the central position toward Te atoms, which central position is slightly off the center and is therefore ferroelectric (see FIG. 4). (3) The unit thus twisted is restored, so that high-speed switching is stably repeated (see FIG. 5).
Note in FIG. 5 that (i) the structure illustrated on the left side corresponds to the structure illustrated in FIG. 3 and (ii) the structure illustrated on the right side corresponds to the structure illustrated in FIG. 4.
Note also that a phase-change material constituted by Ge and Sb without using Te has been recently developed (see, for example, Non patent Literature 5).